1. Field of the Invention
The present invention relates to a novel method of manufacturing diamond films, which can be used for electronic devices and sensors, such as transistors and diodes, heat sinks, surface acoustic wave devices, X-ray windows, optical materials, antiwear materials, decorating materials, and coating all of the above.
2. Discussion of the Background
Diamond is known to have excellent resistance in high temperature environments. It has a large band gap (5.5 eV), and hence is a good electrical insulator in its undoped state. However, it can be made semiconducting by doping with suitable impurity atoms. Furthermore, diamond has excellent electrical properties, such as a high breakdown voltage, high saturation velocities of carriers (electrons and holes), and a low dielectric constant. These characteristics make diamond very suitable for applications in electronic sensors and devices operational at high temperature, high frequency, high electric field, and high power.
Diamond is also expected to be used for various applications, e.g., optical sensors and light emission devices in the short wavelength region, based on the large band gap of diamond; heat sinks based on its high thermal conductivity and small specific heat; surface acoustic wave devices based on its extreme hardness (diamond is the hardest material known); and X-ray windows and optical materials, based on its high transmission and refractive index over a wide range of wavelength from infrared to ultraviolet. Moreover, diamond is used as antiwear parts of many kinds of cutting tools.
In order to fully utilize the excellent characteristics of diamond for these applications, it is important to synthesize high quality diamond films in which structural defects are minimized. As is very well known, bulk diamond crystals are presently produced either by mining natural diamond or by artificially synthesizing bulk crystals under high pressure-high temperature conditions. However, the area of the crystal facets for these diamonds are only about 1 cm.sup.2 at best, and such diamonds are extremely expensive. Therefore, industrial applications of diamond today are limited only to specific fields, such as abrasive powders and high precision cutting tool tips, where small size diamonds are sufficient.
Recently, techniques to deposit diamond films on proper substrates by chemical vapor deposition have been reported. Regarding CVD of diamond films, the following techniques are known: microwave plasma CVD (for example, see Japanese patents (kokoku) Nos. Sho 59-27754 and Sho 61-3320), radio-frequency plasma CVD, hot filament CVD, direct-current plasma CVD, plasma-jet CVD, combustion CVD, and thermal CVD. By these techniques, it is possible to form continuous diamond films over a large area at low cost.
Cathodoluminescence (CL) is known as one of the methods to characterize crystal defects in diamond films. CL is a spectroscopic method to analyze emissions induced by irradiation with a high energy electron beam to samples. Therefore, by CL, emissions of inter-band transitions and emissions attributed to crystal defects and impurities can be detected. Cathodoluminescence is described in detail in The Properties of Diamond, J. E. Field, ed., "Cathodoluminescence", G. Davies, pp. 165-181 (Academic Press, 1979); and The Properties of Natural and Synthetic Diamond, J. E. Field, ed., "Absorption and Luminescence Spectroscopy", C. D. Clark, et al., pp. 59-79 (Academic press, 1992)
FIG. 5 shows the emission spectrum of a conventional CVD diamond film, with cathodoluminiscence intensity (arbitrary unit) on the vertical axis and wavelength horizontally. In conventional CVD diamond films, diamond crystals are oriented in a random fashion and there are many crystal axes on the surface of the diamond film. It is well known that conventional diamond films have a band A emission, which is assigned to crystal defects, in the wavelength region from 350 nm to 700 nm, as shown in FIG. 6 (P. J. Dean, Phys. Rev. Vol. A139, p. 588 (1965)). In this wavelength region from 350 nm to 700 nm, sometimes emission bands are also observed due to impurities, such as nitrogen, which are included in the diamond (J. Walker, Rep. Prog. Phys. Vol. 42, p. 1605 (1979)). It is also well known that band A emission is observed in boron-doped p-type semiconducting diamond films (H. Kawarada, et. al. Vacuum, Vol. 41, p. 885 (1990)).
In cathodoluminescence of a diamond film which possesses perfect crystalline quality and is free of defects, the only emission band observed is at 5.5 eV (225 nm), which is equivalent to the width of the forbidden band of diamond. This is a so-called band edge emission. Band edge emissions are considered to be due to the recombination of electron-hole pairs (excitons), which are generated by irradiation with an electron beam. Band edge emissions were reported to be observed only at low temperatures (liquid nitrogen or liquid helium temperatures) (H. Kawarada et. al., Proceedings of the 2nd International Symposium on Diamond Materials, Electrochemical Society, Penington, N.J., Vol. 91-8, p. 420 (1991)).
There are few reports about detection of band edge emissions at room temperature in conventional diamond films. Essentially, this is because conventional diamond crystals contain a high density of crystal defects and impurities which generate energy levels in the band gap. Given such mid-gap energy levels, the excitons mentioned above have a high probability of recombining through the defect or impurity levels, which are located below the conduction band. Therefore, the emission energy due to the recombination of the excitons is usually much smaller than the band gap energy, which corresponds to the wavelength of 225 nm.
When the energy relaxation process goes through a non-radiative transition, the electron energy is converted to lattice vibrations (phonons) and finally released as heat. The probability of energy conversion to phonons increases with temperature. Therefore, at room temperature, the band edge emissions can not occur or are too weak to be detected.
If a perfect diamond crystal, free of the defect and impurity levels, ever exists, there should be no radiationless transition through such mid-gap levels. It is also expected that the intensity of the band edge emission would not be reduced, even at room temperature. However, it has been difficult to confirm this expectation, because perfect diamond crystals have not been obtained by any conventional methods. Moreover, efforts have not been made to produce such high quality diamond. Thus, the CL measurement at room temperature has not been used as a standard method to analyze the quality of diamond films. It has not been considered that the CL measurement is a suitable technique to determine the quality of diamond films in the process of improving the quality of diamond-based industrial products.
The present invention is proposed in order to solve these problems. It is an object of the present invention to provide high quality diamond films and novel methods for manufacturing such high quality diamond films.